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NSLS-II Beamline Development Proposals Workshops, Soft X-ray Spectromicroscopy, May 20, 2011
Photovoltaic Materials and Devices:
Organic Bulk Heterojunction Solar Cells
Chang-Yong Nam
Electronic Nanomaterials Group
Center for Functional Nanomaterials
• Abundance of solar energy, in the form of light
• Conversion of light into electricity: High cost, low efficiency
• Better understanding and control over material/device parameters:
Can soft x-ray techniques be useful?
Ultimate source of renewable energy: Sun
Light Photovolatics
Sun
Issues of fossil fuel
2
• Inorganic solar cells: High materials and manufacturing costs
• Organic solar cells: Low material & fabrication costs
• Efficiency issue due to its distinctive PV conversion processes
Konarka’s Power Plastic®
Organic solar cells
Konarka’s roll-to-roll processing
Conventional Si solar cells
3
• PV conversion process is “excitonic” : Low dielectric constant
• Exciton diffusion & dissociation at D-A interface
• Current output ~ absorber band-gap
• Voltage (potential energy) output ~ LUMOacceptor – HOMOdonor
• Bilayer device: Low efficiency due to insufficient light absorption
Organic solar cells
LD
Photon
LD: Exciton diffusion length
L~ 5-20 nm
p-type
(donor)
n-type
(acceptor)
Full absorption length:
>200 nm
Energy level diagram under short circuit
4
• Bulk heterojunction: Blend of p- & n-type materials, using solution
• Active layer can be much thicker than exciton diffusion length (LD)
• Best efficiency ~4-5% (P3HT-PCBM) & ~7% (PBDTTT-PCBM)
Organic bulk heterojunction
~LD
Bulk heterojunction
Photon
P3HT (poly-3-
hexylthiophene)
p-type
PCBM
(Derivatized C60)
n-type
L >> LD
Nanoscale phase-separated
morphology
5
US DOE target efficiency
• Multi year plan for
Solar Energy Technology Program
6
DOE target efficiency
• Multi year plan for
Solar Energy Technology Program
7
• Large optical band gap: Not enough light absorption
• Low charge mobility (especially hole): Carrier recombination loss
• Why inefficient charge transport in these materials?
Solar spectrum
Efficiency limiting factors
400 600 800
0.1
0.2
0.3
0.4
(nm)
Ab
so
rba
nce
Optical absorbance
of P3HT:PCBM
Thickness
~135 nm
P3HT: 650 nm
(1.9 eV)
P3HT: 22% coverage
~10% efficiency?
P3HT
8
Charge transport in blend layer
• Complex blend morphology, low crystallinity, hopping, charge traps
• Very low charge mobility : ~10-5-10-3 cm2/V-s
• Significant charge recombination loss
• How to measure charge mobility?
bieff Vμ
L=
2
,ansit timeCarrier tr
? ?
“Dead ends”
9
100
200
300
020
Glazing incidence
x-ray scattering of P3HT
Limited crystallinity in polymer Intermolecular hopping
& charge traps
e.g., ~ 10 s for = 10-5 cm2/V-s
How to measure charge mobility?
• Field effect transistor: “Wrong” orientation, influence of interface
• Time of flight: Thick sample requires, dispersive transport
• Space charge limited current method
10
S D
G
Oxide
Organic semiconductor
VDS
VGS
IDS
A
Back-gated field effect transistor Time of flight measurement
Light pulse
Glass/ITO
Oscilloscope V
P-type, accumulation mode
0
(before saturation)
(after saturation)
or
IDS
VGS
DSOX
VGS
D VL
WC
dV
dI
DS
2
2TGOXDS VV
L
WCI
@VDS
IPhoto
tTOF
Vt
L
TOF
2
t
Photocurrent transient
Space charge limited current
• Bulk limited transport: Built-up charge in organics due to low mobility
• Mott-Gurney law: J ~ V2 with in the proportional constant
• Modification: Effects of traps, electric field
• Example: Mobility in air-processed P3HT:PCBM blend device
11
3
2
8
9
d
VμεεJ so
Metal-insulator-metal,
Low mobility, trap-free
Deep traps, field-dependent detrapping Shallow traps
d
Vμ
d
VJ
d
VJ
ofield
fieldro
effro
891.0exp where
8
9 dependent, Field
8
9 trap,Shallow
3
2
3
2
Space charge limited current
Air-processed P3HT:PCBM blend solar cells
• Spin-coating/contact deposition, ambient air processing
• Post-fabrication vacuum anneal for reduction of oxygen charge trap
ITO/PEDOT:PSS
coated glass
P3HT:PCBM
1-2% Solution Thickness~100-140 nm
1. Spin coating of active layer
Al ~70-100 nm
2. Top metal contact deposition
Nam, Su, et. al., Adv. Func. Mater. 19 (2009)
Device cross-section viewed by TEM
12
Air-processed P3HT:PCBM blend solar cells
• Spin-coating/contact deposition, ambient air processing
• Post-fabrication vacuum anneal for reduction of oxygen charge trap
• Charge mobility through blend network? Nam, Su, et. al., Adv. Func. Mater. 19 (2009)
0.0 0.3 0.6-12
-8
-4
0
4 P3HT:PCBM blend
Curr
ent
density (
mA
/cm
2)
Voltage (V)
Dark
1 Sun, AM1.5G
Efficiency ~4%
Jsc ~9-10 mA/cm2
Voc ~0.65 V
Jsc
Voc
Solar cell current density vs voltage characteristics Device cross-section viewed by TEM
13
P3HT hole mobility in the blend
• Hole mobility: Two conducting channels & selective contacts
• Mobility in 10-5 cm2/V-s range, ~2 orders lower than reported
• Vacuum anneal improves mobility a bit (~20%)
• Poole transport at high bias: NT ~1020 cm-3, oxygen trap at ~0.4 eV
• Nonetheless, decent PV performance
14
Low bias: SCLC High bias: Poole conduction
ITO/PEDOT:PSS
Au
0 1 20
10
20
30
0 2 4 6 8
102
103
104
As-cast
Blend-anneal
JR
1/2 (
A1/2/m
)
Va-V
bi (V)
520
518
517
516
8.0x10-91.0x10
-8
1/F
ln (
J/F
2)
~8.5 V
JR (
A/m
2)
Va-V
bi (V)
(a) (b)
0.1 1 1010
1
102
103
104
2000 4000 600010
1
102
103
104
JF
-R (
A/m
2)
Va-V
bi (V)
JF
-R (
A/m
2)
F0.5
(V0.5
/m0.5
)
~1 V(c) (d)
0 1 20
10
20
30
0 2 4 6 8
102
103
104
As-cast
Blend-anneal
JR
1/2 (
A1/2/m
)
Va-V
bi (V)
520
518
517
516
8.0x10-91.0x10
-8
1/F
ln (
J/F
2)
~8.5 V
JR (
A/m
2)
Va-V
bi (V)
(a) (b)
0.1 1 1010
1
102
103
104
2000 4000 600010
1
102
103
104
JF
-R (
A/m
2)
Va-V
bi (V)
JF
-R (
A/m
2)
F0.5
(V0.5
/m0.5
)
~1 V(c) (d)(c) (d)
Vd
μεεJ so 3
1
8
9d
V
kT
esJ
2exp
Nam, Su, et. al., Adv. Func. Mater. 19 (2009)
Selective electrical contact
P3HT
PCBM
?
Open circuit voltage
• New polymer, PBDTTT: Eg ~1.77 eV (~700 nm), Jsc ~13 mA/cm2
• Improved Voc (~0.76 V) leads to PCE ~ 6.77%
Chen et. al., Nat. Photon. 3 (2009), Solarmer & Yu’s group
15
–3.70
What is open circuit voltage?
• Circuit model vs. energy band model: No driving electric field at Voc
• Voc,max should be 1.4 V or 0.9 V (contact) for P3HT:PCBM
• In reality, Voc ~0.6 V, why?
• Charge recombination should be considered
16
R
I
Light
Solar cell V
V
I
0
Isc
Voc
Simplified circuit model Energy band model: Flat band @ Voc
-3.2 eV
-3.7 eV
-5.1 eV
-6.1 eV
-4.3 eV
-5.2 eV
Al
P3HT
PCBM
PEDOT:
PSS
Quasi Fermi level, Voc, and recombination
• Max potential energy ~ quasi Fermi level difference: qVoc =EFN – EFP
• Steady state charge density n determine Voc
– At steady state: G = R, where G and R are generation and recombination rates
• When light is turned off, Voc decreases due to R: dn/dt = –R = –n/
• Voc transient provides carrier lifetime
17
-3.2 eV
-3.7 eV
-5.1 eV
-6.1 eV
P3HT
PCBM
EV
EC
EFN
EFP
states) ofdensity (eff. ~ and where
exp
exp
VC
FPVV
CFNC
NNpn
kT
EENp
kT
EENn
n
p
qVoc E
q
E
N
n
q
kTV
C
oc ln2
1212
q
kT
dt
dn
nq
kT
dt
dVoc
Preliminary transient Voc data
• Measure voltage required to force zero current under light
• Preliminary vs Voc data, valid only Voc < ~0.3 V
• Linear in semi-log plot: Bimolecular recombination at low n
• If extrapolate, ~10 s at Voc = 0.575 V (full illumination)
18
-0.2 0.0 0.2 0.4 0.6-8
-6
-4
-2
0
J (
mA
/cm
2)
V (V)
1SUN, AM1.5G
PCE = 2.0%
Jsc
= 6.76 mA/cm2
Voc
= 0.575 V
FF = 0.52
2 month old P3HT:PCBM device
0.0 0.5 1.0 1.5 2.0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Off
Vo
c (
V)
t (s)
Light on
0.05 0.10 0.15 0.20
0.01
0.1
Lifetim
e (
s)
Voc
(V)
Voc transient PV characteristics
Lifetime vs Voc
.2
ln 1
ion)recombinatr bimolecula(for and ln2 2
constVkT
qn
nn
Rq
E
N
n
q
kTV
oc
C
oc
n
qVoc E
Bimolecuar
recombination
Important material properties
• Structural & energy characteristics: Crystallinity, molecular
orientation, interfaces, amorphous region, energy levels, electronic
defects
• How to spatially map these parameters and correlate them with
device photovoltaic and electrical performance?
van Bavel, et. al., Adv. Func. Mater. 20 (2009)
Transmission electron microscopy (TEM) images of P3HT:PCBM blend
before and after thermal anneal
Summary
• Organic bulk heterojunction solar cells
– Blend of donor (p-type) and acceptor (n-type) organic semiconductor
• Charge transport and mobility measurement
– Field effect transistor, time of flight, Space charge limited current
– Oxygen effects on P3HT:PCBM blend
• Origin of open circuit voltage and transient measurement
– Carrier lifetime
– Bimolecular recombination
• Acknowledgement
– Electronic Nanomaterials, CFN: Chuck Black, Jon Allen, Danvers Johnston
– Electron Microscopy, CFN: Dong Su
– Condensed Matter Physics: Ben Ocko, Htay Hliang
20